In an Alternating Current (A.C.) ENERGY (J) meter a signal proportional to instantaneous POWER (W) is applied to an INTEGRATOR which outputs a pulse whenever a predetermined unit of energy is measured. The following equations apply: ##EQU1##
FIG. 1 (prior art) shows a typical example of the instantaneous power signal derived from the multiplication of voltage and current signals.
This instantaneous power signal IP as shown in FIG. 1 (prior art) is then typically applied to the input of an integrator as shown in FIG. 2 (prior art). This integrator comprises a capacitor C, a Window Comparator, pulse generation logic and a capacitor discharge switch S.
The power input signal IP is applied to the capacitor C. The capacitor C either charges or discharges to one of two thresholds of the window comparator. If the upper threshold of the window comparator is reached the positive output `.+-.` of the comparator is activated. The Pulse logic circuit outputs a pulse to indicate a unit of forward energy and closes switch S for a set period of time to discharge the capacitor C. Switch S is opened and the process repeats. If the lower threshold of the window comparator is reached the negative output `-` of the comparator is activated. The Pulse logic circuit outputs a pulse to indicate a unit of reverse energy and closes switch S for a set period of time to discharge the capacitor C. Switch S is opened and the process repeats.
One of the disadvantages of this approach is that Direct Current (D.C.) offsets on the IP power signal caused by circuit offsets add or subtract from the measured energy causing system measurement errors. Another disadvantage is the need to use a very low leakage capacitor for the integrating capacitor C to reduce measurement errors caused by the self discharge characteristics of the capacitor. This usually restricts the use of capacitors integrated with the circuit into a silicon chip. One other disadvantage of this approach is the time taken to discharge the capacitor after a unit of energy has been measured again leading, to overall measurement errors.